Passenger cars now have very high performance, because many improvements have been made on engines and other mechanical or electrical elements of the cars. For example, the power output of engines is enhanced and the body weight is lightened. Also, starting, accelerating and turning properties are significantly improved. The progress of such moving properties requires an improvement of braking properties such that cars safely stop under all road conditions i.e., dry roads, wet roads, different friction roads and slippery roads (e.g. ice and snow surface).
In the conventional braking system, on the slippery surface, such as an ice surface tires, are locked immediately when the brakes are applied, and the cars become out of control. This is very dangerous. An anti-lock brake system (ABS) which has been employed since 1978 is a mechanical or electronic system in which the brakes are loosened as soon as the tires are locked. The ABS can safely stop cars under any road conditions without disturbing the car.
On the other hand, FIG. 1 shows a relation between slip ratio of tires and both friction coefficient and cornering force. As is apparent from FIG. 1, the slip ratio indicates a maximum value at a certain point at which braking is most effective. The anti-lock brake system controls the slip ratio of tires to near the maximum value. On the other hand, the cornering force indicates a maximum value at zero slip ratio and reduces as the slip ratio increases. When the tires are completely locked by the brakes (i.e. 100% slip ratio), the cornering force is zero, thus causing the car to be out of control. If both friction coefficient and cornering force are taken into consideration, it is preferred that the maximum value of friction coefficient is present a lower slip ratio in view of the cornering force, but if it is present at very low slip ratio (e.g. less than 10%), it becomes very difficult to control braking. As the result, it is most preferred that a maximum value of the friction coefficient is present at the slip ratio of 10 to 20%, as shown with the diagonal area in FIG. 1. The radial tires which have the above mentioned properties are very suitable for the passengers cars equipped with the anti-lock brake system.